In recent years, closed draw papermaking machines have been developed for achieving improvements in the speed of operation of a papermaking machine. The closed draw papermaking machine does not have an open draw, a part wherein a wet paper web is transferred without being supported in the papermaking process. The closed draw structure solves problems encountered in open draw machines, such as running out of paper. Thus, higher speed operation can be achieved.
A typical closed draw papermaking machine is shown schematically in FIG. 8. A wet paper web WW, shown by a broken line in the figure, is supported by press felts, PF1, PF2, a wet paper web transfer belt TB, and a dryer fabric DF, and is transferred from right to left.
As is generally known, these press felts PF1, PF2, the wet paper web transfer belt TB, and the dryer fabric DF are endless belts, and are supported by guide rollers GR.
A press roll PR, a shoe PS, a shoe press belt SB, and a suction roll SR have structures which are generally known. The shoe PS has a concave shape which conforms with the press roll PR. The shoe PS, the shoe press belt SB, and the press roll PR, form the press part PP.
The wet paper web WW passes successively through a wire part and a first press part, which are not shown, and is then transferred from the press felt PF1 to the press felt PF2. The press felt PF2 transfers the wet paper web to the press part PP. The wet paper web WW, pinched between the press felt PF2 and the wet paper web transfer belt TB, is compressed by the shoe PS, and the press roll PR, having the shoe press belt SB therebetween. The press felt PF2 has high water permeability, and the wet paper web transfer belt TB has little or no water permeability. Therefore, the water in the wet paper web WW moves to the press felt PF2 at the press part PP. Immediately after the press felt PF2, the wet paper web WW, and the wet paper web transfer belt TB, move out of the press part, the pressure is suddenly released and their volume expands. This expansion, and the capillary phenomenon exhibited by the pulp fibers forming the wet paper web WW, cause rewetting of the web WW, in which part of the water in the press felt PF2 moves to the wet paper web WW.
Since the wet paper web transfer belt TB has very low permeability, it does not hold water. Therefore, rewetting does not occur in the wet paper web transfer belt TB, and thus, the wet paper web transfer belt TB contributes to improvement in the efficiency of water removal from the wet paper web.
After the wet paper web WW moves out of the press part PP, it is transferred by the wet paper web transfer belt TB, and is sucked onto the suction roll SR and transferred to a drying process by a dryer fabric DF.
The wet paper web transfer belt TB is required to transfer a wet paper web WW while the web is attached to the belt after moving out of the press part PP, and to allow smooth removal the wet paper web from the transfer belt when he wet paper web is transferred to the next stage of the process. Conventionally, various structures have been proposed for realizing these functions. For example, U.S. Pat. No. 4,529,643 discloses a structure where a needle felt, comprising a woven fabric and a batt fiber intertwiningly integrated with the woven fabric by needle punching, is impregnated with a high molecular weight elastic material and cured. U.S. Pat. No. 4,500,588 discloses another structure which is shown in FIG. 9. In FIG. 9, a wet paper web transfer belt TB10 comprises a woven fabric 31, a batt fiber 41 intertwiningly integrated with the woven fabric 31 by needle punching, and a high molecular weight elastic section 51 provided in the batt fiber 41. This transfer belt TB10 has a wet paper web side layer TB11 and a machine side layer TB12, and is characterized in that the surface of the wet paper web side layer TB11 does not have a high molecular weight elastic section 51 and comprises only batt fibers 41.
Japanese Patent No. 3264461 discloses a further structure as shown in FIG. 10. This wet paper web transfer belt TB20 comprises a woven fabric 31, a high molecular weight elastic section 51, formed on one side of the woven fabric 31, and a batt layer 41 formed on the other side of the woven fabric 31. Therefore, the wet paper web side layer TB21 of the wet paper web transfer belt TB20 is formed by the high molecular weight elastic section 51 and a machine side layer TB22 is formed by the batt layer 41.
The surface of the wet paper web side layer TB21 is made rough, for example, by grinding. This surface has a structure wherein its surface roughness, according to JIS-B0601, a ten-point average roughness Rz, is in the range from, 0 to 20 microns when the belt is in the press part, and in the range of 2 to 80 microns after the belt moves out of the press part.
The ten-point average roughness Rz, in the range of 0 to 20 micron when the belt is in the press part, is maintained immediately after the belt moves out of the press part. In other words, the surface of the wet paper web side layer TB21 is smooth at this point. Therefore, a thin water film can be formed between the wet paper web and the surface of the wet paper web side layer TB21. The wet paper web is suitably attached to the surface of the wet paper web side layer TB21 due to the adhesion of the thin water film.
When the wet paper web transfer belt TB20 travels further, the ten-point average roughness Rz is in the range of 2 to 80 micron. Thus, the thin water film between the wet paper web and the surface of the wet paper web side layer TB21 is broken, and the adhesion is reduced. Therefore, the transfer of the wet paper web to the next stage of the process becomes easy. In other words, the wet paper web transfer belt TB20 shown in FIG. 10 and disclosed in Japanese Patent No. 3264461 realizes the function necessary for a wet paper web transfer belt.
In the case of the wet paper web transfer belt of U.S. Pat. No. 4,529,643, voids between the batt fibers are not always filled up with the high molecular weight elastic section. On the other hand, in the case of the structure of the U.S. Pat. No. 4,500,588, the wet paper web side layer is formed only by the batt layer. In both cases, the wet paper web side layer is formed by the batt layer. Therefore, in the case of these wet paper web transfer belts, a large amount of water is absorbed in the wet paper web side layer and thus, rewetting occurs occasionally. In addition, the function of transferring a wet paper web by attaching it to a transfer belt, and the function of removing the wet paper web from the transfer belt smoothly when the wet paper web is transferred to the next stage of the process, cannot be fully realized.
The wet paper web transfer belt of Japanese Patent No. 3264461 is intended to utilize the change in surface roughness caused by compression, and release of compression, of the wet paper web side layer, so that a filler breaks the water film between the wet paper and the surface of the transfer belt, web. However we have determined from testing that the filler has a reverse effect. Because of capillary action, the rough surface has a high ability to hold water between the wet paper web transfer belt and the wet paper web. Consequently, the function of smoothly transferring the wet paper to the next stage of the papermaking process is not fully realized, and thus problems are encountered in the papermaking process.
In view of the above problems, it is an object of the invention to provide a wet paper web transfer belt, in which a wet paper web is reliably transferred by attachment to a transfer belt, and the wet paper web is smoothly and reliably removed from the transfer belt for transfer to a next stage of the papermaking process.